Interactions and Chemical Reactivities Between a Janus Tubular Aluminosilicate and the Cement Matrix—Case of Nuclear Waste Stabilization

This work deals with the conditioning of non-incinerable radioactive oil, issued from the nuclear industry, and which is still deprived of an appropriate treatment. The investigated strategy consists in using Janus particles with dual affinities for cement and oil to encapsulate oil in cement.<br/>The chosen Janus particles are hybrid nano-clays (hybrid aluminosilicate imogolites) with tubular shape[1]. Their Janus character come from the specific properties of their internal and external surfaces. The internal surface of the nanotubes being covered by Si-CH₃ groups, this one is hydrophobic and can trap and stabilize hydrophobic compounds. The external surface formed by alumina surface is on the contrary hydrophilic and positively charged up to pH~10. It thus has good affinity for the mixing water used for cement hydration.<br/><br/>In this presentation, after a brief demonstration of the favorable interaction between oil and imogolite[2], we will focus on the impact of hybrid imogolites in the cement formulation with respect to oil incorporation and Portland cement hydration[3].<br/><br/>Previous research work shows that Pickering emulsions can be formed between oils and hybrid imogolites[2]. The stabilization mechanism is a two steps process. The first one consists in a fast stabilization of the water/oil interfaces by the nanotubes, controlled by the hybrid imogolites adsorption (pendant droplet technique). The second step shows that the adsorption allows to form a solid/gel layer that evolves slowly with time generating a stable gel structure between the drops. Direct Pickering emulsions incorporate oil up to 60% by volume by only adding a mass fraction of hybrid imogolite materials lower than 1 wt%. These emulsions stabilized by an original Janus particle have a typical behavior of both gel and Pickering emulsion, allowing to efficiently encapsulating oils.<br/><br/>Hexadecane was used as a non-radioactive surrogate of the contaminated oil. Hydration of cement pastes with variable contents of imogolites and / or oil was monitored using isothermal microcalorimetry. Unlike hexadecane, hybrid imogolites were shown to retard the hydration process. The delay depended both on the imogolite and water contents. To understand the mechanism involved in this delay, a simplified system, consisting in hybrid imogolites and tricalcium silicate 3CaO.SiO₂, the main constituent of Portland cement, was thoroughly investigated. Hydration of tricalcium silicate was stopped by the solvent displacement method after increasing periods of time and the solid fraction of the paste samples was characterized by solid state NMR (²⁹Si and ²⁷Al), XRD and TGA. Imogolites were progressively destabilized under the highly alkaline conditions set by the cement paste, releasing aluminate ions to the solution, which were consumed to precipitate an aluminum-substituted calcium silicate hydrate (referred as C-A-S-H phase). This early precipitation of C-A-S-H could be responsible for the delay in cement hydration, since it is known that C-A-S-H particles do not act as nucleation and growth sites for further calcium silicate hydrate (C-S-H)[4,5]<br/>The role of this C-A-S-H phase formation for the affinity with oil of the hydrated hybrid cement paste is still to be explored.<br/><br/>Acknowledgments: The authors gratefully acknowledge the French Alternative Energies and Atomic Energy Commission (CEA) and the FOCUS research program for funding.<br/><br/>[1] P.D.G. Cradwick, et al., Imogolite, a Hydrated Aluminium Silicate of Tubular Structure, Nat Phy Sci. 240 (1972) 187–189.<br/>[2] E.Puel et al., Pickering emulsions stabilized by Janus nanotubes: oil triggers an evolving solid interfacial layer<i>. </i>J. Col Int Sci, to be submitted (2022).<br/>[3] E.Puel et al., to be submitted (2023).<br/>[4] F. Begarin et al., Hydration of alite containing aluminium, Adv App Cer. 110 (2011) 127–130.<br/>[5] D. Wagner et al., Influence of aluminium on the hydration of triclinic C3S with addition of KOH solution, Cem Conc Res. 137 (2020) 106198.